Human LCA, the most severe form of degenerative retinal disease, is thought to be caused by abnormal differentiation or extremely premature degeneration of photoreceptor cells. However, with the exception of a very few types, little is known about the mechanisms underlying the developmental defects of LCA, mainly due to limited access to human embryonic LCA tissues and the lack of suitable animal models that faithfully recapitulate the various phenotypes of heterogeneous LCA diseases. LCA8, an inherited retinal dystrophy that begins during the fetal stages, is caused by mutations in Crb1, one of the three mammalian homologues of Drosophila Crumbs. Unexpectedly, the Crb1 knock-out mouse does not recapitulate human LCA8, most likely due to functional compensation by the closely related homologues Crb2 and Crb3. Because CRB and PALS1, a bona-fide CRB interacting partner, are functionally interdependent, and Pals1 exists as a single gene in the mouse genome, we hypothesize that deleting Pals1 will completely inhibit the function of the CRB proteins, thus phenocopying the Crb1 null mutations identified in LCA8 patients. In this study, we propose to create a conditional knock-out of Pals1 in mice to establish a human LCA8-like mouse model to elucidate disease-causing mechanisms at the cellular and molecular levels during embryonic development and adult stages. Furthermore, we will attempt to rescue the Pals1-deficient retinas via perinatal transplantation of retinal stem cells. Preliminary results show that Pals1CKO retinas exhibit novel embryonic phenotypes such as increased retinal cell proliferation and disorganized retinal lamination as well as postnatal LCA8-like phenotypes, including severe photoreceptor degeneration and concomitant loss of ERG in young adults. This project will establish the novel concept that the loss of Pals1 function in retinal progenitor cells is a causative pathologic event that leads directly to the establishment of LCA lesions. In addition, this research project will greatly enhance the understanding of PALS1 and CRB polarity complex protein function in the developing retina and the role of polarity gene defects during LCA8 development. Furthermore, the early developmental defects induced in Pals1CKO mice provide a unique opportunity to test the feasibility of and to optimize perinatal cell transplantation therapy, which offers a promising approach to restoring the vision lost during development by replacing diseased cells with normal retinal stem cells.